Question

这怎么可能：

mysql> select id from posts;
+----+
| id |
+----+
|  1 |
+----+
1 row in set (0.00 sec)

mysql> select id from tags;
+----+
| id |
+----+
|  1 |
+----+
1 row in set (0.00 sec)

mysql> insert into pots_x_tags values(1,1);
ERROR 1452 (23000): Cannot add or update a child row: a foreign key constraint fails (`posts_x_tags`, CONSTRAINT `posts_x_tags_ibfk_2` FOREIGN KEY (`tag_id`) REFERENCES `tag` (`id`) ON DELETE CASCADE)

以下是表格（多对多）：

CREATE TABLE `post_tag_map` (
  `post_id` int(11) NOT NULL,
  `tag_id` int(11) NOT NULL,
  PRIMARY KEY (`post_id`,`tag_id`),
  FOREIGN KEY (`post_id`) REFERENCES posts(`id`) ON DELETE CASCADE,
  FOREIGN KEY (`tag_id`) REFERENCES tag(`id`) ON DELETE CASCADE
)  
CREATE TABLE `tags` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `tag` varchar(255) NOT NULL,
  PRIMARY KEY (`id`),
  UNIQUE (`tag`)
)
CREATE TABLE `posts` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `title` varchar(255) CHARACTER SET latin1 NOT NULL,
  `body` text CHARACTER SET latin1,
  PRIMARY KEY (`id`)
)

Answer 1

从表面上看，post_tag_map.tag_id列上的外键似乎指向名为tag的表，而不是名为tags的表。

MySQL外键约束失败

1 个答案: